The invention relates to a method for preparing synchronization of a motor-generator set for synchronization with a power grid, wherein a DC motor is connected by a shaft with a three-phase generator, with a photovoltaic system as a primary energy source for the DC motor, wherein the three-phase generator can be connected to a power grid. The invention also relates to an apparatus for carrying out the method.
Such method is suitable to feed the AC current or three-phase current generated by the three-phase generator in a large system to a power grid, preferably a public grid.
Photovoltaic systems are known where the entire DC current supplied by the photovoltaic modules is converted with an electric converter or inverter into AC current or three-phase current, which is then fed into the power grid. Currently, electric converters for large facilities are commercially available, which include semiconductor components and which have a power rating of up to 700 kW. Of course, these are rather expensive. For a system having an even greater power output, several electric inverters must be provided. For example, a modern solar facility with a capacity of 2.5 MW includes at least nine such electric inverters, with each of them having a capacity of 330 kW.
Although an electric inverter has a high-efficiency for energy conversion, it is known to react rather sluggishly due to the presence of capacitors with a rather large electric capacitance. Currently, such inverter requires between 20 seconds and three minutes before its control device can adapt to changes in incident solar energy.
It has been proposed in DE 10 2006 026 073 to employ a DC motor in combination with an AC generator driven by a common shaft (motor-generator set). DC motors and AC generators for such high power ratings are commercially available. Such combination is advantageous compared to using a plurality of electric inverters, because they are statistically less likely to be affected by failures. Another advantage is that less maintenance is required. It is also advantageous that only a single unit must be monitored during operation and that the service life of an electromechanical drive set can be many times that of electronic elements with proper maintenance.
DE 20 2006 002 726 U1, which forms the basis of this invention, discloses a motor-generator set operating in a solar facility. This publication, however, only addresses the mechanical aspects of the motor-generator set. It contemplates several pole wheels which are arranged on a common shaft, each having its own stator.
When operating this type of facility with the corresponding method, a clean and uninterrupted adaptation of the output parameters of the three-phase generator to the grid has proven to be complicated, which is necessary for connecting the three-phase generator to the power grid. This can be attributed to the fact that the corresponding control valuables change continually with the incident light intensity. With such “fidgety” input variables, achieving an acceptable switching time, where undesirable compensation currents may be tolerable, can take considerable time. Such compensation currents cause stress and hence undesirable aging of the affected components. Disadvantageously, generated solar energy cannot be supplied to the power grid during the adaptation process, i.e., until satisfactory connection conditions are attained.